l. Field Of The Invention
This invention relates to circuit breakers and hot wire relays for use in protecting a circuit from high leakage current or short circuits.
2. Description Of The Prior Art
The present invention relates to a time delay disconnect switch or hot wire relay as one component of a ground fault circuit interrupter for use with an appliance such as an electric range for the purpose of preventing high current leakage hazards or short circuits. Such ground fault circuit interrupters (GFCI) are required by the Underwriters Laboratory, Inc. for use with bathrooms, outdoor residential receptacles, swimming pools, and construction sites. An example of such a ground fault circuit interrupter system is taught in the Clark Pat. No. 3,895,263. Such a system employs a differential current transformer for detecting a leakage current path by measuring the difference in current entering the load and in the current leaving the load, and if the signal from the sensor exceeds the level detector threshold, it is amplified and timed. If the signal is above the threshold long enough to time out the timer, a driver will supply power to the disconnect switch which will interrupt power to the load. This particular patent does not describe in detail the particular disconnect switch that it employs.
The present invention employs a disconnect switch in the form of a hot wire relay of a special design to obtain time delay. An example of a simple hot wire relay is shown in the Bergsma Pat. No. 3,176,099 which uses an in-circuit self-heating resistance wire referred to as a "hot" wire or "sag" wire such that when the magnitude of the input circuit current exceeds a certain limit, the hot wire will elongate sufficiently for making or breaking the output circuit-controlling electrical contact.
Another example of a snap-action hot wire power-switching relay is described in the Staples U.S. Pat. No. 3,514,733, which is assigned to the assignee of the present invention. One of the main differences between the present invention and the Staples relay is the construction to provide time delay in the present disconnect switch to avoid spurious operation and destructive arcing and premature failure.
The Doyle U.S. Pat. No. 3,864,649 shows a ground fault circuit interrupter and its disconnect switch combined in a single wiring device, while in the present invention the disconnect switch is separate from the sensing and firing circuit.
The Brackett U.S. Pat. No. 3,142,737 shows a hot wire relay or circuit breaker having a positive connection between the movable contact blade and the manula reset plunger. This patented design is much more complex and expensive than the present invention. The value of this patented design can only be judged fairly by its operation under actual test conditions, but its complexity over the present invention would lead one skilled in this art to doubt its acceptability.
The present disconnect switch employs wiping contact action which serves to maintain the contact surfaces clean, but this is not generally new per se, as it is taught in the Sway U.S. Pat. No. 2,671,840 and the Hottenroth U.S. Pat. No. 2,315,960, both of which are assigned to the present assignee. It is felt that the present invention relates to the overall design of an adjustable time delay disconnect switch and the novel method of obtaining the wiping action of the contacts and avoiding destructive arcs during contact opening.
The principal object of the present invention is to provide an adjustable time delay disconnect switch for use in a ground fault circuit interrupter system with a time delay that can be varied between 15 milli-seconds and 500 milli-seconds, depending upon the circuit conditions to be governed.
A further object of the present invention is to provide a GFCI with a disconnect switch having switch contacts with a controlled contact gap in less than a half cycle so as to restrict the amount of arc struck during contact separation.